1. Field of the Invention
The present invention relates to the formation of nonwoven webs from thermoplastic polymers. More particularly, it relates to webs formed by meltblowing. This process is used primarily to form thermoplastic microfibers and involves spinning a molten polymer and contacting it while molten with a fluid, usually air, directed so as to form filaments or fibers and attenuate them. After cooling, the fibers are collected and bonded to form an integrated web. Such webs of microfibers have found particular utility as filter materials, absorbent materials, moisture barriers, and insulators. In achieving high speed production of such materials, it is important that the polymer viscosity be maintained low enough to flow and prevent plugging of the die tip which will normally require that the polymer be heated. Further, high quality products and webs require that uniformity and strength properties be maintained at desired levels.
2. Description of the Prior Art
Early work in the formation of meltblown microfibers is described in various government publications relating to work done by the Naval Research Laboratory in Washington, D.C. Examples include NRL Report 4364 "Manufacture of Super-Fine Organic Fibers" by V. A. Wendt, E. L. Boon, and C. D. Fluharty; NRL Report 5265 "An Improved Device for the Formation of Super-Fine Thermoplastic Fibers" by K. D. Lawrence, R. T. Lukas, and J. A. Young. The process described uses an adjustable extruder to force a hot thermoplastic melt through a row of fine orifices into high velocity dual streams of heated gas, usually air. The nozzle design provides for immediate resumption of attenuation following breaks which occur at sub-micron dimensions. Through the control of air and nozzle temperatures, air pressure, and polymer feed rate, fiber diameters may be regulated. Preparation of fabrics from these fine fibers is also disclosed. Improvements to this process are described in many patents including, for example, U.S. Pat. No. 3,676,242 to Prentice issued July 11, 1972; U.S. Pat. No. 3,755,527 to Keller et al issued Aug. 28, 1973; U.S. Pat. No. 3,825,379 to Lohkamp et al issued July 23, 1974; U.S. Pat. No. 3,849,241 to Buntin et al issued Nov. 19, 1974; and U.S. Pat. No. 3,825,380 to Harding et al issued July 23, 1974. In all such disclosures it is contemplated that the molten polymer be attenuated by a stream of hot, inert fluid, usually air. Forming webs in such cases usually requires forming distances of at least about 12 inches to provide for fiber forming, cooling and attenuation. Such distances frequently result in undesirable non-uniformities in the web and its properties. At shorter forming distances a harsh, stiff web is often produced with a preponderance of "shot" or solid polymer globules.
It is also known to provide insulation on the outer surface of spinning dies to reduce heat loss into the surrounding environment. For example, U.S. Pat. No. 2,571,457 to Ladisch issued Oct. 16, 1951 discloses such an insulated die. It has, moreover, been suggested that in certain cases spun fibers may be contacted by cold gas to accelerate cooling and solidification. For example, U.S. Pat. No. 4,112,159 to Pall issued Sept. 5, 1978 contains such a disclosure. However, it remains a desired goal to improve the formation of meltblown nonwoven fabrics and to achieve further economies in processes and apparatus used to form such fabrics.